Rate based on historical models. It slows down. This guy assumes it just keeps growing at the same rate. Europe has shown it slows down and the US is starting to. Italy and Spain are no longer doubling every few days but actually decreasing daily cases. The reasons behind this could be many herd immunity, better treatments,weakening strains, weather, mitigation factors etc..
I was hoping Csouls projections were close and after tracking Europe for the last few weeks I have a lot more confidence the hope is justified.